1. Field of the Invention
The present invention relates to a back-flow prevention apparatus.
2. Description of the Related Art
Conventionally, an injection molding machine has an injection unit. The injection unit has a heating cylinder in which a screw is disposed rotatably and in an advancingly-retreatively movable manner. Drive means rotates and advances or retreats the screw. In a metering step, the screw is retreated while being rotated in a regular direction, so that resin drops from a hopper and is melted and stored in a space located ahead of a screw head. In an injection step, the screw is advanced so as to inject the resin melt into a mold from an injection nozzle.
FIG. 1 shows a longitudinal sectional view of a main portion of a conventional injection unit.
In FIG. 1, reference numeral 11 denotes a heating cylinder. The heating cylinder 11 has an injection nozzle 13 at its front end (left-hand end in FIG. 1). In the heating cylinder 11, a screw 12 is disposed rotatably and in an advancingly-retreatively movable manner. Unillustrated drive means rotates and advances or retreats the screw 12.
The screw 12 extends rearward (to the right in FIG. 1) within the heating cylinder 11. The screw 12 is connected at its rear end to the drive means and has a screw head 14 at its front end. A spiral flight 15 is formed on the surface of a metering portion 18 to thereby form a groove 16 along the flight 15.
In the thus-configured injection unit, in a metering step, the drive means is activated so as to retreat (move to the right in FIG. 1) the screw 12 while rotating the screw 12 in a regular direction. Resin pellets contained in an unillustrated hopper enter the heating cylinder 11 and are advanced (moved to the left in FIG. 1) through the groove 16. While being advanced through the groove 16, resin pellets are melted by an unillustrated heater, and resin melt is stored in a space located ahead of the screw head 14.
In an injection step, the drive means is activated so as to advance the screw 12. The resin stored in the space located ahead of the screw head 14 is injected into an unillustrated mold cavity from the injection nozzle 13, thereby filling the cavity.
In order to prevent backflow of the resin stored in the space located ahead of the screw head 14 in the injection step, a back-flow prevention apparatus is disposed.
Specifically, the screw head 14 has a conical head body portion 21 formed at its front section and a small-diameter portion 19 formed at its rear section. An annular back-flow prevention ring 20 is disposed around the circumference of the small-diameter portion 19, thereby defining a resin passageway 24 between the small-diameter portion 19 and the back-flow prevention ring 20. A seal ring 22 is disposed at the front end of the metering portion 18 such that the seal ring 22 can contact or separate from the rear end of the back-flow prevention ring 20.
Accordingly, in the injection step, when the screw 12 is advanced, the resin stored in the space located ahead of the screw head 14 is urged to move rearward, i.e., to flow rearward. However, resin pressure causes the back-flow prevention ring 20 to move rearward with respect to the screw 12. Thus, the rear end of the back-flow prevention ring 20 abuts the seal ring 22 to thereby effect sealing. As a result, the resin stored in the space located ahead of the screw head 14 is prevented from flowing rearward.
In contrast, in the metering step, when the screw 12 is retreated while being rotated in a regular direction, resin pressure causes the back-flow prevention ring 20 to move forward with respect to the screw 12. Thus, the front end of the back-flow prevention ring 20 abuts the rear end of the head body portion 21. Since axially extending cuts 25 are formed in the head body portion 21 in a plurality of circumferential positions, resin flow is not hindered.
However, in the above-mentioned conventional back-flow prevention apparatus, when suck-back is performed after the metering step is completed and before the injection step is started, the resin contained in the metering portion 18 is moved forward into the space located ahead of the screw head 14, causing variation in resin quantity stored in the space located ahead of the screw head 14.
Also, since through advancement of the screw 12 the flow-back prevention ring 20 is moved rearward with respect to the screw 12 to thereby effect sealing, sealing timing varies depending on the state of kneading and dispersion of resin, resin viscosity, resin temperature, and acceleration to a predetermined speed of the screw 12 at the time of starting the injection step. Accordingly, the quantity of back-flow resin varies.
Thus, there is devised a back-flow prevention apparatus which prevents resin movement associated with suck-back and variation in timing for effecting sealing.
FIG. 2 shows a longitudinal sectional view of a main portion of another conventional injection unit.
In FIG. 2, reference numeral 32 denotes a screw which is disposed rotatably and in an advancingly-retreatively movable manner in an unillustrated heating cylinder. The screw 32 includes a metering portion 33 and a screw head 34 disposed at the tip end of the metering portion 33. A spiral flight 35 is formed on the surface of the metering portion 33 to thereby form a groove 36 along the flight 35. An internal-thread portion 42 is formed at the front end (left-hand end in FIG. 2) of the metering portion 33.
The screw head 34 includes a conical tip portion 37 formed at its front end, a cylindrical body portion 38 formed at its central section, a seal ring portion 39 formed at its rear section and in the form of a flange integral to the body portion 38, and an external-thread portion 41 formed at its rear end. Through screw-engagement between the external-thread portion 41 and the internal-thread portion 42, the screw head 34 can be fixedly attached to the metering portion 33. First resin passageways 45 are formed through the seal ring portion 39 between the front end face of the portion 39 and the rear end face of the portion 39 at a plurality of circumferential positions.
An annular back-flow prevention ring 43 is disposed around the circumference of the body portion 38 such that the rear end of the ring 43 is in the proximity of or in contact with the front ends of the first resin passageways 45. Second resin passageways 46 are formed through the back-flow prevention ring 43 between the front end face of the ring 43 and the rear end face of the ring 43 at a plurality of circumferential positions. A pin 51 is disposed through the body portion 38 at a predetermined position so as to stop rotation of the back-flow prevention ring 43, to bring the rear end of the back-flow ring 43 in the proximity of or into contact with the front end of the seal ring portion 39, and to prevent the back-flow ring 43 from coming off the screw head 34.
Arc engagement grooves 52 are formed in a front end portion of the back-flow prevention ring 43 at positions corresponding to the pin 51, so that both ends of the pin 51 engage the grooves 52. Each of the engagement grooves 52 is formed over a predetermined circumferential angle .theta.. As both ends of the pin 51 move along the engagement grooves 52, the screw head 34 and the back-flow prevention ring 43 rotate relative to each other by the angle .theta.. The first resin passageways 45 and the second resin passageways 46 are formed such that the passageways 45 are circumferentially shifted from the passageways 46 by the angle .theta.. Accordingly, as the screw head 34 and the back-flow prevention ring 43 are rotated relative to each other, the ring 43 assumes either a communication position where the first resin passageways 45 and the second resin passageways 46 align with each other to establish communication therebetween or a shutoff position where the communication between the passageways 45 and 46 is broken.
In the thus-configured back-flow prevention apparatus of an injection unit, in a metering step, when the screw 32 is retreated while being rotated in a regular direction, the screw head 34 is rotated in the same direction as is the screw 32. Accordingly, the seal ring portion 39 and the pin 51 are rotated in the same direction as is the screw 32. However, the back-flow prevention ring 43 remains stationary until the pin 51 abuts end walls of the engagement grooves 52. When the pin 51 abuts the end walls of the engagement grooves 52, the back-flow prevention ring 43 is in the communication position where communication is established between the first resin passageways 45 and the second resin passageways 46. Subsequently, the back-flow prevention ring 43 is rotated in the same direction as is the screw 32.
Accordingly, as the screw 32 is retreated while being rotated in the regular direction, resin contained in the metering portion 33 moves forward through the first and second resin passageways 45 and 46 and is thus stored in a space located ahead of the screw head 34.
When the metering step is completed as above, suck-back is to be performed. Before suck-back is performed, the screw 32 is rotated in a reverse direction by a predetermined angle not smaller than the angle .theta.. As a result, the screw head 34 is rotated in the same direction as is the screw 32. Accordingly, the seal ring portion 39 and the pin 51 are rotated in the same direction as is the screw 32. However, the backflow prevention ring 43 remains stationary until the pin 51 abuts the other end walls of the engagement grooves 52. When the pin 51 abuts the other end walls of the engagement grooves 52, the back-flow prevention ring 43 is in the shutoff position where the communication between the first resin passageways 45 and the second resin passageways 46 is broken. Subsequently, the back-flow prevention ring 43 is rotated in the same direction as is the screw 32.
Next, suck-back is performed so as to reduce resin pressure within the space located ahead of the screw head 34, thereby preventing the resin stored in the space from oozing through the tip of an unillustrated injection nozzle. At this time, even when the screw 32 is retreated, resin contained in the metering portion 33 does not move forward, thus preventing variation in resin quantity stored in the space located ahead of the screw head 34.
In the subsequent injection step, when the screw 32 is advanced, the resin stored in the space located ahead of the screw head 34 is urged to flow rearward. However, since the back-flow prevention ring 43 is in the shutoff position, the communication between the first resin passageways 45 and the second resin passageways 46 is broken.
As a result, the resin stored in the space located ahead of the screw head 34 can be prevented from flowing rearward. Thus, the quantity of resin to be injected remains constant, thereby preventing molded products from suffering short shot, burrs, or like defects.
In the thus-configured back-flow prevention apparatus, when the back-flow prevention ring 43 is in the communication position, communication is established between the first resin passageways 45 and the second resin passageways 46; and when the back-flow prevention ring 43 is in the shutoff position, the communication between the passageways 45 and 46 is broken. In order to properly effect the communication/shutoff function, the second resin passageways 46 must be adequately positioned in a circumferential direction with respect to the first resin passageways 45. This positioning can be easily attained through integral formation of the seal ring portion 39 with the body portion 38 and through adequate selection of position of a hole formed in the body portion 38 for receiving the pin 51.
However, the integral formation of the seal ring portion 39 with the body portion 38 involves a drawback that in the case of breakage of the seal ring portion 39, the entire screw head 34 must be replaced, resulting in higher cost.